Robert Putnam is variously described as looking like Abraham Lincoln or an Amish preacher. The 66-year-old American social scientist bears more than a physical resemblance to such figures; his wisdom has earned him a wide audience - from the White House (under both presidents Clinton and Bush) to the bestseller lists, while still managing to maintain an unequalled academic respect in his field.

"Social capital" - our networks of friendship, neighbourhood and trust - was an obscure sociology concept until Putnam managed to draw out its political implications and write about it accessibly in his book Bowling Alone.

Now, at a time when some academics might be thinking of graceful retirement, Putnam is working on at least three major books as well as directing large programmes of research in the US and in a joint Harvard/ Manchester University collaboration on social change. In the course of an interview, he ranges widely from the role of religion in US society to changes in family life and the rise of inequality.

But top of his mind when we meet in Manchester, is immigration. With great trepidation, he has just published his first, much awaited, paper on his five-year study of social capital in the US - the biggest survey of its kind - which concludes that ethnic diversity does reduce social capital. He found that the higher the diversity in a neighbourhood, the lower the levels of trust, political participation and happiness between and within the ethnic groups, and he called it "hunkering". But what has prompted criticism is not his analysis of hunkering, which the right has seized upon with delight, but his optimistic assertion that this is a short-term problem that, with "intelligence and creativity", can be overcome.

"Diversity is a social construction that can be deconstructed and reconstructed - you can erase a line and draw a new line [to define identity] and we do it all the time," says Putnam, who adds that there has been much more response to his research in the UK than in the US.

"Some critics [in the UK] on the right say that's all hogwash. What gets the conservatives irritated is that I say the task is not to 'make them like us' but to create a new 'we' - a new, more encompassing identity. They say: 'Why should we? We don't want a new we, we like the old we.' But in the US, we don't have that problem because we have changed in the past," he says.

Much of his optimism is rooted in his reading of US history and how an ethnonationalism of white Anglo Saxon Protestant America was transformed into a civic nationalism as a response to mass immigration of late 19th century. But he agrees that national identity is never static; the US now faces the issue of how Hispanic it is willing to become in the wake in recent decades of a huge influx of Latinos. Such arguments make Putnam sympathetic to recent thinking in the UK about Britishness from figures such as Gordon Brown and Trevor Phillips - both of whom have called him in to discuss his research.

"Civic education is the right way to go; it's hard to create a sense of national identity without acknowledging some shared things. Not everything has to be shared - colour and god can be different - but there has to be something in common. Brown is trying to identify what we all share when he talks about British values - that's the right direction," says Putnam.

It is easy to see why politicians like dealing with Putnam - the list of political leaders who have called him to hear about his work stretches across the political spectrum from Bill Clinton to George Bush, even to Libyan leader Muammar Gadafy - because he has huge reserves of optimism. His faith in the ingenuity of human beings to tackle and resolve problems is strong, and infectious.

"Big social change defines the modern world - industrial revolution, women's revolution, internet revolution - we rightly use the word revolution because these changes are not about switching the model, they are much bigger than that," he says. "Some are driven by technology and some by values. They are always controversial because they have good and bad consequences - they always create winners and losers.

"The question is how to minimise the very real costs. The problem is that the transition costs come early - the benefits take longer. Take, for example, the industrial revolution, which lowered life expectancy in the short term. But no one suggested that the people crowding into cities such as Manchester should go back to their villages. The local government in Manchester worked out the public health systems needed, and the world copied."

It is the public health reformers of British provincial cities such as Manchester that inspired Putnam and are an important part of the reason why he sited the US/UK comparative research project in the city. He sees his work as directly comparable to those 19th century city reformers - collect the empirical data, then work out solutions.

The Harvard/Manchester programme has identified four initial areas of social change on which to focus: immigration; the changing workplace and the consequences of women moving into the paid workforce; the changing role of religion in society; and inequality, particularly the mounting evidence of the inheritance of class and how it restricts social mobility.

Steep declines

Across many of these issues, Putnam's old interest, social capital, features. As he demonstrated in Bowling Alone, social capital has a crucial impact on crime reduction, educational achievement, even life expectancy. His research had exposed steep declines in all forms of social capital across much of the developed world, which he detailed in Bowling Alone with its central image of the end of US bowling leagues, but Putnam maintains he is "optimistic about social capital".

What fascinates him is tracking where the new forms of social capital are developing and why they are successful. One of his key areas of interest is religion - religious affiliations account for half of all US social capital. He cites US megachurches which, typically, attract tens of thousands of members, as "the most interesting social invention of late 20th century."

He identifies the secret of their success: "They have very low barriers to entry - the doors are open, there are folding chairs out on the patio - they make it very easy to surf by. You can leave easily. But then they ramp people up to a huge commitment - at some megachurches, half of all members are tithing [giving a tenth of their income]. How do they get from the low to the high commitment? By a honeycomb structure of thousands of small groups: they have the mountain bikers for God group, the volleyball players for God, the breast cancer survivors for God, the spouses of the breast cancer survivors for God, and so on.

"The intense tie is not to the theology but in the emotional commitment to others in their small group. Most of these people are seeking meaning in their lives but they are also seeking friends. The small groups spend two hours a week together - doing the volleyball or the mountain biking and praying; they become your closest friends," he says.

"These churches form in places of high mobility - people live there for six weeks and the church provides the community connection. When you lose your job, they'll tide you over, when your wife gets ill, they'll bring the chicken soup."

Putnam believes that this low entry/ honeycomb structure could be successfully copied to reinvigorate many other organisations, from trade unions to scouts' clubs and rotary clubs. He points out that the leader of the US's biggest trade union, the Service Employees Union International, is intrigued by the potential of the megachurch model.

The other fast developing area of social capital is on the internet. Putnam has been studiedly cautious about the impact of the internet and insists its too early to be definitive: "We've got to get beyond the [notion that] the internet is good or bad for social capital. What is interesting is how it can be used to encourage 'alloyed networks' - which are both cyber and face to face - like email.

"I think strong social capital has to have a physical reality - a purely virtual tie is a pretty thin reed on which to build anything; it's highly vulnerable to anonymity and spoofing and very difficult to build trust. But I'm a member of Facebook, the social networking site, and it enables me to keep up with old students; it has the potential to be both positive and meaningless - I get notices from people all over the world asking me to be my friend on Facebook but what does that mean?'

What could be really interesting, says Putnam, is what would happen if you put the model of megachurches together with social networking - that could produce new and powerful forms of organisation. He introduced the leader of one of the biggest megachurches to the founder of the meetup.com, which connects people with others in their neighbourhood with similar interests. It is a very successful website but not "sticky" in internet terms, people do not keep up with the site, whereas megachurches are extremely successful at sticky so he is curious whether the two types of social organisation might be able to complement each other.

Force for good

The project that is most pressing on Putnam's time is a book he is writing on religion, American Grace. He credits religion with a vital role in spurring on progressive change in the US over the past 150 years - contrary to popular European wisdom, religion has predominantly been a force for good in America and its current use by the political right is an aberration from its history, he argues.

"Religious revival has been an essential ingredient of every progressive movement: the abolition of slavery came out of the second great awakening; the progressive era of 1900-15 when the US first passed labour and environment legislation came out of the social gospel movement," says Putnam, who is a convert to Judaism.

Much of his book will be devoted to analysing how that progressive potential in religion was lured to the Republican right. It is easy to see how his interest in social capital and religion fit together, but he is quick to acknowledge that religion can also have detrimental consequences, and it is possible to have social capital that has no religious underpinning.

He strikes a warning on the secularisation of Europe, which he describes as the first large-scale effort to see whether secular progressive countries can reproduce themselves and successfully pass on the values on which they were built. "I believe they can," he says, "but the evidence is not yet in. Europe is still living off its religious heritage."

One of the most frequent criticisms of Putnam is that his thinking is steeped in a misplaced nostalgia for the 1950s. It is a criticism he is quick to reject, arguing on two distinct fronts: first, that there were some very positive aspects to the 50s which got an unfair billing from the 60s radicals and cannot be dismissed simply as nostalgia. Second, that he has no interest in reversing social changes such as the arrival of television or the move of women into paid employment.

What interests Putnam is an honest appraisal of the losses incurred by these kinds of social change; and in a neat aside, he points out that the Luddites were right to protest, but landed on the wrong solutions. Only after careful rigorous research, can the full ingenuity of human adaptation be mobilised to minimise those costs.

Career 1979-present: Peter and Isabel Malkin professor of public policy, Harvard University; visiting professor and director of the graduate summer programme in social change, Manchester University; 1975-79: professor, University of Michigan.

Publications 2003: Better Together: Restoring the American Community (with others); 2000: Bowling Alone: The Collapse and Revival of American Community.

Public life Member, National Academy of Sciences; fellow, British Academy; 2001-02: president, American Political Science Association; 1978-79: on the staff of the National Security Council.